A translation that meets the criterion of fidelity (faithfulness) is said to be "faithful"; a translation that meets the criterion of transparency, "idiomatic". Depending on the given translation, the two qualities may not be mutually exclusive.
The criteria for judging the fidelity of a translation vary according to the subject, type and use of the text, its literary qualities, its social or historical context, etc.
The criteria for judging the transparency of a translation appear more straightforward: an unidiomatic translation "sounds wrong"; and, in the extreme case of word-for-word translations generated by many machine-translation systems, often results in patent nonsense.
Nevertheless, in certain contexts a translator may consciously seek to produce a literal translation. Translators of literary, religious or historic texts often adhere as closely as possible to the source text, stretching the limits of the target language to produce an unidiomatic text. A translator may adopt expressions from the source language in order to provide "local color".
Current Western translation practice is dominated by the dual concepts of "fidelity" and "transparency". This has not always been the case, however; there have been periods, especially in pre-Classical Rome and in the 18th century, when many translators stepped beyond the bounds of translation proper into the realm of adaptation.